The background of the invention is the need in the petroleum industry for cost reducing subsea operation with an equal or higher level of safety, compared to the present practice. It is commonly known that developing and operating a subsea field that typically has multiple wells with associated Xmas trees requires a large investment both in equipment and operational costs. A large portion of the cost for such a development stems from the drilling, completion, startup and service of the wells. Traditionally the industry has used large drilling rigs with appurtenant drilling system for drilling towards a reservoir, and then installing a subsea wellhead and well casings. When these have been installed, an Xmas tree (well head module) is landed on the wellhead in order to control the production after starting up. It has been common to also install the Xmas tree from the drilling rig. Starting up the well is typically performed with so-called workover systems (service systems) which are connected to the Xmas tree and which provide a mechanical access from the drilling rig to the subsea well and reservoir. It will be possible to run an internal work tool on a steel wire (wireline operation) or a small work pipe (coiled tubing—typically a 2″ pipe) down into the well, by means of a workover system, in order to pull plugs and to open to reservoirs for production. Such a workover system can also be used for service in the well and for control or optimization of the production throughout the lifetime of the well. Common for such operations and systems is that they involve large costs to manufacture, operate and maintain.
Accordingly there exists a demand for solutions that are useful for installation and testing of subsea Xmas trees, as well as well service, without the use of a drilling rig. This technology or equipment shall therefore make it possible to move such operations to lighter vessels or ships which are not necessarily required to handle hydrocarbons up to the vessel deck. It will also be advantageous to let the drilling rig perform the operation for which it is optimized, namely to drill the well and to install casings and tubing. This will result in a more efficient use of the drilling rig, since it then does not need to shift between the types of risers which are in use. The logistic aboard the rig will also become easier if the Xmas tree is not taken aboard, since this requires both space and handling of large weights (typically 30-50 tons). The heavy weight and the size of a workover system is also significant, as such a system includes many containers on deck, as well as large reels with umbilicals.
It will be desirable to introduce new technology which reduces the operational costs, has less weight and size, and does not expose personnel to equipment exposed to well pressure. This will provide reduced requirements to the lifting and handling equipment on the vessel, as well as improved HSE. A main reason for the large weight of drilling and workover systems is the requirement of cutting the drilling or work string that penetrates the barrier envelopes if needed. An example of this will be at loss of well control, where the main safety valves must be activated to shut in or isolate the well from the environment. Such safety valves are typically known as elements in a blowout preventer (BOP) or a lower riser package (LRP). The weight of a BOP can typically be from 200 tons to 500 tons, while an LRP typically has a weight of 30 tons to 50 tons. An object of the present invention is to flush a main bore in an installation and intervention tool where the barrier envelopes are not penetrated, so that the requirement of cutting functionality can be avoided. This will again result in significantly lighter equipment for maintaining well control. Another desirable object is to avoid well exposure directly up to the vessel. This also includes avoiding potential direct or indirect leakage paths through attached hoses and pipes (umbilicals). It is essential that the tool does not contain pressurized hydrocarbons when it is placed on the vessel, before or after the well operation, so that less requirements are raised for such handling aboard.
Alternative systems for flushing have been suggested, and the nearest known art is described in the patent publications NO330819, NO309439, WO2011/039514, and US2011/0192610. Common for these solutions is that they will potentially expose the operation vessel to hydrocarbons—either directly by carrying along filled tanks or indirectly via leakages through attached umbilicals or pipes. Their use also requires that the suggested systems operate together with a well control package (lower riser package/well control package), as the known art discusses use in association with a subsea lubricator where a tool string (wire) penetrates through a pressure control head (pack box and grease injector head). This implies the inclusion of a closing and cutting valve on the lower side of the pressure control head as a result of the penetration.
The object of the invention is to make a closed well operation possible. With closed well operation is meant activities down in a subsea well without the use of cable or coiled tubing that penetrates well barriers, or use of riser from well to vessel.